1. Field of the Invention
The present invention relates to an objective lens having a high numerical aperture (NA) for high-density optical focusing, an optical pickup adopting the objective lens, and a relatively thin optical disk suitable for the optical pickup.
2. Description of the Related Art
The information recording and reproduction density in an optical disk increases as the size of a light spot focused on the optical disk by an optical pickup decreases. In general, the size of the light spot focused on an optical disk is proportional to a wavelength (λ) of a light source over a numerical aperture (NA) of an objective lens. Thus, as shown in a functional relationship (1) below, the shorter the λ of the light source and the larger the NA of the objective lens, the smaller the size of the light spot.size of light spot∝λ/NA  (1)
For a higher recording and reproduction density, an optical pickup needs a light source capable of emitting a shorter wavelength of light and an objective lens having a high NA. However, due to the limitation in manufacturing a single objective lens, it is impossible to manufacture an objective lens having an NA of 0.8 or higher, making it difficult to satisfy the need for an allowable error below an optical aberration of 0.07λrms. A conventional optical pickup for information recording and reproduction and objective lens is illustrated in FIGS. 1 and 2.
The conventional optical pickup illustrated is capable of recoding information with high density over an optical disk 1 having a 0.1-mm thick protective layer. The optical pickup includes a light source 11 having a wavelength, λ, of 400 nm, a grating 19 diffracting and transmitting an incident beam, a first polarization beam splitter (PBS) 21 altering the traveling path of the incident beam in a predetermined polarization direction, a λ/4 plate 23 guiding a circular polarized beam to the optical disk 1, and an objective lens unit 50 having an NA of 0.85. The optical pickup further includes a second PBS 27 transmitting or reflecting the incident beam from the optical disk 1 and, subsequently, from the first PBS 21. A main photodetector 31 receives the incident beam passed through the second PBS 27 and detects an information signal from the incident beam. A servo photodetector 37 receives the beam reflected from the second PBS 27 and detects an error signal therefrom.
The optical pickup further includes a collimating lens 13 collimating the incident beam, a beam shaping prism 15 shaping the incident beam, and a λ/2 plate 17 delaying the phase of the incident beam. The collimating lens 13, the beam shaping prism 15, and the λ/2 plate 17 are arranged on the optical path between the light source 11 and the grating 19. A second λ/2 plate 25 delaying the phase of the incident beam is further disposed on the optical path between the first PBS 21 and the second PBS 17. A first condensing lens 29 condenses the incident parallel beam and it is arranged between the second PBS 27 and the main photodetector 31. A second condensing lens 33 condenses the incident parallel beam and an astigmatism lens 35 creates astigmatism. The second condensing lens 13 and the astigmatism lens 13 are arranged between the second PBS 27 and the servo photodetector 37. A monitoring photodetector 41 monitors the optical power of the light source 11 from the beam reflected by the first PBS 21 and condensed by a third condensing lens 39. The objective lens unit 50 includes an objective lens 51 focusing the incident beam and a semi-spherical lens 55, which is arranged between the objective lens 51 and the optical disk 1, to increase the NA of the objective lens unit 50. In the above configuration of the objective lens unit 50, an NA of 0.6 can be secured by the objective lens 51 and increased by the semispherical lens 55.
Referring to FIG. 2, as long as the semispherical lens 55 does not cause an incident beam to refract, the NA of the semispherical lens 55 is proportional to the product of sin θ and a refractive index, n, of the semispherical lens 55, wherein θ is the maximum incident angle θ of light into the semispherical lens 55, which is expressed by equation (2). Thus, the NA of the objective lens unit 50 can be increased up to 0.85.
 NA=n sin θ  (2)
However, the conventional optical pickup includes two lenses to achieve such high NA. Thus, if a tilting occurs between the semispherical lens 55 and the objective lens 51, keeping a low optical aberration becomes difficult. When the semi-spherical lens 55 and the objective lens 51 are assembled into the objective lens unit 50, a restrictive control of distance and tilting error between the semispherical lens 50 and the objective lens 51 is needed, thereby making mass production of the objective lens unit 50 difficult.
In manufacturing the optical disk 1, an error in thickness is 3% or more. Accordingly, if the optical disk 1 has a thickness of 0.1-mm, the thickness error is ±3 μm or more. Such thickness error creates serious coma aberration and astigmatism when the objective lens unit 50 has an NA of 0.8 or more. Thus, a restrictive error management is desired in manufacturing a 0.1 mm thick optical disk so that the thickness error is within ±3 μm. However, it is difficult to produce a 0.1 mm thick optical disk with ±3 μm thickness error, or with a maximum thickness error of 5 μm, on a large scale.
In the conventional optical pickup described above, aberration caused by an error in thickness of the optical disk 1 is corrected by adjusting distance between the objective lens 51 and the semispherical lens 55. However, the configuration of an actuator for adjusting the distance between the objective lens 51 and the semispherical lens 55 is complicated.